(a) Field of the Invention
This invention relates to a continuous mining device for cobalt rich crust deposits existing on relatively narrow sea-bottoms of the seamounts and a method of operating a mining ship which mounts said device. This invention further relates to an improved continuous mining device for mineral resources existing in the deep sea-bottom such as phosphorate ore or manganese nodule and a mining method using the device.
Crust deposit is a type of deep sea manganese oxide metallic deposit but is basically different from manganese nodule deposits existing on the surface of 4,000 to 6,000 m deep sea bottoms which have been the major subject of research for the last fifteen years.
Crust deposits lie on the sea bottoms of a less depth than the manganese nodule deposits and are distributed on slopes of seamounts or flat terraces like the sea bottoms of 800 m through 2,000 m depth. Unlike manganese nodules, they do not become deposited over a large area of the deep sea bottom but lie on relatively restricted areas. As their ore density is extremely high, mining crust deposits require a unique technique which can effectively mine ores within a narrow area. The device and method according to this invention improve the conventional continuous mining device for crusts for better practical use and provide a method for using the device with optimal efficiency.
(b) Description of the Prior Art
The experiment conducted by the present inventors in 1972 at the depth of 4,900 m in the waters south-east of Hawaii with the ship Daini Kyokuyo Maru (17,000 t) was the first mining experiment for the deep sea metallic nodules. The test was conducted on the method of mining by using one-ship sideward towing system which is shown in FIG. 6. The system attempts to prevent a rope from becoming entangled as their ends are separated. CNEXO of France proposed the two ship headward towing system which is shown in FIG. 7. The method attempts to prevent tanglement of a rope by positioning the two ships apart from each other by an appropriate distance. These systems were proposed to overcome the problem of rope tanglement encountered in the test using the Daini Kyokuyo Maru.
The present inventors conducted another test on the one ship headward towing system shown in FIG. 8 in the waters off the Bonin Islands in 1975. This system attempted to separate both ends of a rope from each other with hydrodynamic drag force applied on separator boards attached to the rope while the ship is being driven in the direction of the bow at an extremely slow speed. In FIGS. 6, 7, and 8, the letter S denotes a ship, the arrow marks the direction of movement of the ship, 4 a rope, and B buckets. The letter P denotes a board for separting ends of a rope with hydrodynamic drag force. The inventors filed a patent application for a continuous mining device for crust deposits in Japan on Oct. 12, 1985 as Japanese Patent Application Number 60 - 225973. The application was based fundamentally upon the device which realizes the above three systems.
In the test conducted in the summer of 1972 in the waters off the Hawaiian Islands with the Daini Kyokuyo Maru, crust deposits on the seamounts were actually mined by the continuous method of one ship sideward towing method shown in FIG. 6.
The crust was mined continuously from the foot of a seamount in the depth of ca. 4,800 m for about 120 buckets. The test proved that the continuous line bucket method was more suitable for crust deposit mining from the seamounts or other rugged sea bottoms than the suction pump mining method. In this test, we experienced rope tanglement about 3 times, and so how to prevent rope tanglement became the most important problem for continuous line bucket. The two ship headward towing system in FIG. 7 and one ship headward towing system in FIG. 8 were invented by France CNEXO and us after the Hawaii test.
This history of development of continuous line bucket mining was reported by Masuda at Hiro Hawaii conference which related the beginning of deep sea mining. The above three methods, however, were found with the following defects respectively.
In the one ship sideward towing system, a power device such as a side thruster must additionally be mounted on the mining ship for moving the ship sideward. This thereby pushes up the costs of the device, as well as the fuel costs for power. Towing a ship sideward is a difficult operation especially above the deposits near the seamounts where the deposit area is narrow.
As the two ship headward towing system needs two ships, communications and coordinated operation are difficult, thereby creating problems in the mining operation. Further, the costs of the mining operation inevitably increase as the system requires two ships.
The third system or the one ship headward towing system seems most feasible, but according to the prior art system, it requires a number of separation boards which act as separators attached on a rope at an interval. The attaching/detaching operation of such boards should be conducted continuously during the mining operation and thus proves too much trouble to employ during offshore operation.
Though all of these three systems may be adapted as a method for continuous mining device for crust deposits, there has been keenly felt a necessity to urgently develop an improved arrangement which is safer and more economical, and which is practically feasible.
Unlike manganese nodules, crust deposits lie in small and narrow areas in the limited seamounts where the ocean currents and conditions thereon constantly undergo complicated variations. In order to operate a ship over such area, the method should fully satisfy the condition that the operation be simple, and also it must prevent rope tanglement to separate two rope lines by some method.